The present invention is related to pillared trioctahedral-type natural micas and vermiculites, to a preparation method thereof, and to their applications.
1. Technical Background of the Invention
Pillared interlayered smectites (PILCs) with a large variety of pillars have been described in the scientific literature (journals, patents), among which the Al-pillared clays are the most documented ones. Similar materials with pillars based on other elements such as Zr, Cr, Ti, Si, Fe, Ga, Si, Ta, V, Mo, Nb, combinations of two or more of these elements or combinations of one or several of those elements with others elements not mentioned above (as e.g. Ni, Cu, Co, etc.), rare-earth (La, Ce . . . )-containing pillars have been successfully prepared and reported in the literature. Pillared clays containing two or more elements in the pillars are also named mixed pillared clays.
Pillared clays show interesting potentialities in catalysis, as catalysts or supports to catalytic phase(s) or in admixture with other catalysts or catalyst components (e.g. zeolites, metal oxides, etc.), especially as catalysts for e.g. hydrocarbons transformation. Pillared materials also find potential interest as adsorbents and in other domains such as in gas separation processes; as scavengers for heavy metals (treatment of waste water); in SO2 and NOx abatement; in purification of edible oil, cation selective composite membranes; as solid electrolytes; as host materials for (conducting) polymers; etc.
Trioctahedral Micas
Trioctahedral micas refer to layered 2:1 sheet (or lamellar) silicates in which the octahedral layer is sandwiched between two adjacent tetrahedral layers and mainly contains divalent cations with the results that all the possible octahedral positions are occupied. They differ from dioctahedral micas (muscovite-type), where ⅔ of the octahedral positions are filled with mostly trivalent cations. The general formula of the end-member phlogopite mineral is K2Mg6(Si6Al2)O20(OH,F)4. The structural substitutions mainly occur in the octahedral layers but also in the tetrahedral ones and are responsible for the wide range of chemical compositions of the trioctahedral micas. The high number of substitutions is at the origin of the high net negative layer charge in micas. Potassium is usually the dominant interlayer cation ensuring electroneutrality of the layers. Trioctahedral micas may contain substantial amounts of fluorine (replacing structural hydroxyls) which conveys resistance to weathering, hardness and thermal resistance. The principal cations in the octahedral layer of natural trioctahedral micas are Mg2+, Fe2+, Al3+ and Fe3+, with smaller proportions of Mn2+, Ti4+ and Li+. Phlogopites refer to trioctahedral micas in which more than 70% of the occupied octahedral sites contain Mg2+, whereas biotites define the micas where 20 to 60% of these sites are Mg2+ [Newman and Brown, in Chemistry of Clays and Clay Minerals, A. C. D Newman (Ed.), Mineralogical Soc. 6, Longman, 1987, p. 75]. The potassium ions located between the unit layers just fit into hexagonal cavities (perforations) in the oxygen plane of the tetrahedral layers. Adjacent layers are stacked in such a way that the potassium ion is equidistant from 12 oxygens, 6 of each tetahedral layer [R. E. Grim, Clay Mineralogy, McGraw-Hill, 1953, p.65]. In their original state, natural micas do not swell in the presence of water or polar solvents because the hydration energy of the interlayer potassium ions is insufficient to overcome the co-operative structural forces at the coherent edges of a cleavage surface [Newman and Brown, Nature 223, 175, 1969].
The absence of swelling properties of natural micas makes it impossible, without modifying the mineral, to obtain pillared intercalated forms equivalent to those readily obtained with swelling clays (smectites) in which the clay sheets are separated from each other by pillars of inorganic nature, which confer to these materials thermally resistant structural and textural characteristics such as permanent elevated spacings, high specific surface area and micropore volume, and surface properties (acido-basic, redox).
Vermiculites
Vermiculites belong to a group of hydrated aluminium silicates. These minerals may be considered as xe2x80x9cswelling trioctahedral micasxe2x80x9d containing Al-for-Si substitutions in the tetrahedral layers (as in micas), and Al-, Fe-, and Ti-for-Mg substitutions in the octahedral layers. Because of both types of substitutions, the overall negative charge of the structure results, as in micas, from an imbalance between the negative charge of the tetrahedral layer and the excess positive charge of the octahedral layer. As in micas and smectites, the excess negative charge is counterbalanced by cations located in the region between adjacent sheets which ensure electroneutrality of the layers. Most often, the interlayer cations are magnesium ions. The layer charge densities in vermiculites are intermediate between those of micas and smectites. Unlike micas, vermiculites may swell and the layers may expand when polar molecules are introduced in the interlamellar region but this swelling capability is much reduced compared with smectites. The interlayer charge balancing cations (magnesium ions) are exchangeable.
Vermiculites (and a fortiori micas) could not be intercalated with bulky poly-hydroxy-aluminum species to form a pillared material exhibiting spacings of about 17-18 xc3x85 (gallery height of about 8 xc3x85) as in pillared smectites, a failure which has been attributed to the high layer charge density of these minerals. Contacting vermiculite suspensions with Al13-containing pillaring solutions led to expanded materials exhibiting only about 14 xc3x85 spacings [references 1-7]. Taking advantage of the high spacings (27-28 xc3x85) developed upon adsorption of long chain amines and alcohols to introduce Al pillars was unsuccessful [reference 5]. Preliminary dealumination of vermiculite by treatment with an aqueous solution of (NH4)2SiF6 followed by the addition of the pillaring solution did not result in materials with improved spacings [reference 7]. A mixture of a pillared fraction of vermiculite (with 18 xc3x85 spacing stable at 500xc2x0 C.) and of unpillared fraction was obtained upon contacting with Al13-containing solutions a suspension of vermiculite that was previously treated with L-ornithine [reference 8]. However, repeated attempts to reproduce the method were unsuccessful.
2. State of the Art
The documents U.S. Pat. Nos. 5,200,378 and 5,017,537 are concerned with the pillaring of synthetic layered phosphates. Layered phosphates have nothing in common with natural micas. The intercalation is performed after a previous intercalation of an amine (amide or dimethyl sulfoxide) in order to expand the interlayers. Attempts to pre-swell vermiculite with a long chain amine or alcohol and to treat the expanded vermiculite with a pillaring solution did not allow to obtain 18 xc3x85 Al-pillared vermiculite.
The documents U.S. Pat. No. 5,340,657 and EP-0240359 deal with the Al-pillaring of synthetic sodium tetrasilicic fluor micas which have nothing in common with natural micas. The Na-TSF micas have only octahedral substitutions (Li for Mg or Mg for Al), but no aluminium in the tetrahedral layers. Natural micas have substitutions in both the tetrahedral (Al for Si) and octahedral (Al, Fe for Mg) layers. Na-TSF micas are synthesized in a soda-containing medium (thus no interlayer potassium as in natural micas). The presence of exchangeable Na in the interlayers as charge neutralizing cations confers swelling properties. Natural micas have potassium ions between the layers and do not swell in polar media. Na-TSF micas can be pillared when they are contacted with the pillaring solution. Nothing like occurs when doing so with natural micas. This is the principal reason for the prerequisited conditioning operation of the natural micas (aiming at the charge reduction of vermiculites and micas and conversion to homoionic form of hydrated ions). Synthetic Na-micas have, as hydrothermally synthetic layer materials, very small particle sizes. Particles of the order of 0.1 micron are preferred in the document EP-0240359 (p. 3, lines 8-10).
The document U.S. Pat. No. 4,510,257 describes a method which allows to intercalate three-dimensional silicon oxide pillars from organo-silicon derivatives in the clay interlayers. The material is then calcined to decompose the organic moiety. Vermiculite is mentioned (yet no example of successful Si-pillared material is provided).
The document WO98/00091 deals with the pillaring of synthetic layered silicate materials which have no octahedral layers and are thus different from either synthetic sodium fluor tetrasilicic xe2x80x9cmicasxe2x80x9d or natural micas (as in our patent application), both of which having octahedral layers.
This invention describes a method for the obtention of pillared trioctahedral-type micas (PILMs) and vermiculites (PILVs) characterised by thermally stable interlayer distances, high specific surface areas and micropore volumes, and acidic properties. These features are similar to those found for equivalent pillared interlayered materials obtained from naturally occurring swelling clays, or smectites, (or their hydrothermally synthesised analogues) such as montmorillonites (bentonites), beidellites, hectorites (fluorhectorite and laponite, synthetic analogues), saponites, nontronites, rectorites (interstratified montmorillonite-muscovite), Ni-SMM and SMM (the so-called synthetic expandable mica-montmorillonite) to quote some of the main ones used in the preparation of pillared interlayered clays (PILCs).
Pillaring is achieved after submitting the starting micas and vermiculites to a conditioning procedure consisting of chemical and thermal treatments which aim to reduce the layer charge density and replace the charge balancing potassium ions located in the interlayers of the initial micas, or the magnesium ions in the case of vermiculites, by hydrated cations such as f.i. sodium ions. The charge reduced cation-exchanged (Na+, Ca+2, . . . ) forms of micas and vermiculites may be converted to any other cationic form(s) by simple exchange of the interlayer cations (fi. Na+) by the desired element(s). Pillared micas and vermiculites are obtained by contacting Na-micas and Na-vermiculites with solutions containing the pillaring species, namely, polyoxohydroxymetal cations which intercalate between the layers according to a cation-exchange process, in a similar manner as for the obtention of pillared smectites. Successful insertion of Al-polymerised species is not restricted to the sole Al element. Substitution of Al in the pillaring solution by any one of the elements indicated below or mixtures thereof which have been successfully employed in the preparation of pillared smectites, give rise to equivalent pillared micas and vermiculites, thus offering materials with a wide variety of intercalated pillars and mixed pillars differing in the nature of the pillaring species and composition.
It is one object of the present invention that the same preparation procedure may be equally applied to trioctahedral micas and vermiculites and wastes thereof (as defined below) to obtain pillared materials exhibiting the characteristic features of analogous materials prepared from smectites.
In accordance with the aforernentioned objectives, it is a particular object of the invention to find a new route to the pillaring of trioctahedral micas and vermiculites with solutions containing Al hydroxy-polymeric species often referred to as AlO4Al12(OH)24(H2O)127+ (in short, Al13) with Keggin-like structure [reference 9]. This objective is realized through the partial reduction of the layer charge density, which may be compared to an xe2x80x9caccelerated weatheringxe2x80x9d process, and through the application of pillaring solutions in the form of partially hydrolysed Al solutions, the Al species in presence in these solutions having been identified [references 9-12].
It is a further object that this invention is not restricted to the sole case of aluminium as the metal element of the pillar since, as stated above, substitution of Al in the pillaring solution by anyone of the elements Zr, Ti, Si, Cr, Fe, Ta, Nb, Ga etc. or combinations of different elements including lanthanides or mixtures thereof give rise to equivalent pillared micas and vermiculites.
Therefore, it is an object of the invention to give access via the successful Al-pillaring of micas and vermiculites to the preparation of materials with different types of pillar species (based, e.g., on Zr, Ti, Si, Cr, Fe, Ta, Nb, Ga, etc, or combinations of different elements, including lanthanides) with possible uses in various catalytic reactions and other application areas.
Further, the greater intrinsic structural stability of micas and vermiculites compared with smectites is of considerable interest in achieving pillared materials which possess improved resistance to thermal treatments, a weakness shared by all smectite-based pillared materials.
Another interest of the method is the possibility to use micas and vermiculites with various particle sizes.
Other objects of the invention include post-exchange and/or impregnation of the pillared materials, improvement of the acidic properties, use in fluidised bed applications.
Further details will appear in the claims and in the description hereafter of preferred embodiments of the invention.